The most distinctive property of a superconductive material is its absence of electrical resistance when it is at or below a critical temperature. This critical temperature (Tc) is an intrinsic property of the material.
Research into the ability of specific materials to superconduct began in 1911 with the discovery that mercury superconducts at a Tc of about 40xc2x0 K. Since then, many applications for superconducting materials have been conceived, but such applications could not be commercialized because of the extreme low Tc of the superconducting materials then available.
Although many materials have since been examined in an effort to find compositions which superconduct at higher temperaturesxe2x80x94temperatures at which the more economical and practical coolant of liquid nitrogen could be usedxe2x80x94until about 1986 the highest temperature superconductor known was Nb3Ge having a critical temperature, Tc, of approximately 23.2K. Before 1987, superconducting devices, even those which employed the Nb3Ge superconductor, required the use of liquid helium as the refrigerant-coolant.
In late 1986 Bednorz and Muller disclosed that certain mixed phase compositions of Laxe2x80x94Baxe2x80x94Cuxe2x80x94O appeared to exhibit superconductivity being at an onset temperature, Tco, of about 30K. Bednorz et al., Z. Phys. B., Condensed Matter, Vol. 64, pp. 189-198 (1986). Investigation of that Laxe2x80x94Baxe2x80x94Cuxe2x80x94O mixed phase system established that the crystalline phase therein responsible for superconductivity had a crystal structure like that of K2NiF4 (214). Since then it has been determined that whatever might be the rare earth metal or the alkaline earth metal constituent of a 214 system, the upper temperature limit of superconducting onset, Tco, of superconductors of a 214 type crystalline structure is no greater than about 38K. Liquid helium was still required as the coolant for such a 214 type of material.
Following the discovery of superconductivity in a rare earth-alkaline earth-Cu oxide system of a 214 crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which are superconductive at temperatures above the boiling point of liquid nitrogen, 77K. These new rare earth-alkaline earth-copper oxides are of the formula L1M2Cu3O7 wherein L is a rare earth metal and M is an alkaline earth metal. The L1M2Cu3O7 compositions are commonly referred to as xe2x80x9c123xe2x80x9d high-temperature superconductors in reference to the stoichiometry in which the rare earth, alkaline earth, and copper metal atoms are present, namely a ratio of 1:2:3. Subsequent to the discovery of the 123 high temperature superconductors, another form of high temperature superconductor was discovered having the formula T2Mxe2x80x22CanCun+1O6+2n wherein T is bismuth and Mxe2x80x2 is strontium or T is thallium and Mxe2x80x2 is barium, and xe2x80x9cnxe2x80x9d is 1, 2, or 3. Both types of xe2x80x9chigh temperature superconductingxe2x80x9d (HTS) compositions are ceramics materials.
The 123 high temperature superconducting compounds have a perovskite related crystalline structure. The unit cell of such 123 compounds consists of three sub-cells in alignment along the crystallographic C-axis wherein the center of the middle subcell is occupied by a rare earth metal atom, the center of each end subcell is occupied by an alkaline earth metal atom, and copper atoms occupy the corner positions in each subcell. X-ray and neutron powder diffraction studies indicate the structure of superconductive 123 compounds to be oxygen deficient and that the ordering of oxygen in the basal planes is critical to the existence of superconducting properties in such compounds. See C. Poole et al, Copper Oxide Superconductors (John Wiley and Sons 1988). The unit cell formula of a 123 compound is L1M2Cu3O6+xcex4(xcex4=0.1 to 1.0, preferably about 1.0) wherein the rare earth metal constituent, L, is yttrium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium, or mixtures thereof including mixtures with scandium, cerium, praseodymium, terbium and the alkaline earth constituent, M, is barium, strontium or mixtures thereof. Studies indicate that when xcex4 is between about 0.1 to about 0.6, the resulting 123 compound assumes a tetragonal unit cell crystallographic symmetry and is non-superconductive. In the tetragonal unit cell symmetry, the lattice dimension of the C-axis is approximately 11.94 angstroms and that of the A and B axis is approximately 3.9 angstroms. When xcex4 is between about 0.7 and 1.0, the resulting 123 compound has an orthorhombic unit cell crystallographic symmetry and is superconductive. The orientation of the oxygen atoms in the unit cell causes, the unit cell to compress slightly along the A crystallographic axis and thus the lattice dimension of the A axis is less than that of the B axis. Lattice constants in the orthorhombic symmetry are about A=3.82, B=3.89 and C=11.55 angstroms.
With the discovery of the new xe2x80x9chigh temperature superconductingxe2x80x9d (HTS) compoundsxe2x80x94HTS compounds are those which superconduct at a Tc above the temperature at which liquid N2 can be used as a refrigerantxe2x80x94it has become economically possible to pursue many previously conceived applications of the superconductivity phenomena which before were commercially difficult wherein cooling by liquid helium was required. Since they superconduct at temperatures greater than 77K, the new high temperature superconductors may in practical applications be cooled with liquid nitrogenxe2x80x94a more economically feasible refrigerant. The HTS compounds, both the 123 compositions and those containing bismuth or thallium, simplify and enhance the reliability of commercial applications of superconductors. Recent studies also show that the HTS compounds have better performance at 4K than the prior materials.
Nevertheless, the ceramic HTS compounds have been economically and technologically impractical for use in some applications due to the inability of bodies thereof (1) to carry high current loads (Jc(0)) (2) to carry high current loads in intense magnetic fields (Jc(H)) (3) and to entrap strong magnetic fields (Bt). As a result, significant commercial and technological barriers against use of the ceramic HTS compounds as a superconductor body in a variety of practical applications, such as in magnets, magnetic separators, transmission lines, trapped field magnets, levitation bearing and magnetically levitating trains (meglav), still exist.
In magnetic separators, for example, the body of a superconducting material is required, as a practical constraint, to have a critical current density (Jc) between about 103 to 105 amps/cm2 in a magnetic field between 0 to 10 T. To be practical for some magnet applications, bodies of a ceramic HTS compound must be capable of entrapping within its crystalline structure a high magnetic field. The critical current (Jc) which a body of a HTS compound is capable of carrying is strongly affected by the granular alignment and homogeneity of the polycrystals HTS compound comprising the body and by the distribution and force with which lattice defects within the HTS material can pin magnetic flux lines. Accordingly, one approach to improve the Jc of a ceramic HTS body has been directed to methods of processing the HTS composition into shaped bodies wherein the number and content of xe2x80x9cweak linksxe2x80x9d due to its granular ceramic nature is reduced. Another approach has examined techniques whereby strong flux pinning centers may be homogeneously introduced into the HTS composition of which a body article is composed.
The Jc and ability to entrap a magnetic field of a HTS compound body, particularly a 123 HTS compound, is dramatically influenced by several factors which introduces xe2x80x9cweak linksxe2x80x9d into the HTS material of the body. xe2x80x9cWeak linksxe2x80x9d exist in the forms of (1) grain boundaries; (2) micro-cracks; (3) impurity contentxe2x80x94i.e., the wt. % content of the body of nonsuperconducting phases (i.e., L2BaCuO5, BaCuO2, CuO, etc.); (4) the porosity of the body (i.e. body density); (5) chemical inhomogeneity; and (6) electrical anisotropy.
A xe2x80x9cweak linkxe2x80x9d is any non-superconducting defect which intervenes between the electrical communication from one part of an HTS body to an adjacent part of an HTS body. A 123 HTS compound crystallizes into xe2x80x9cgrainsxe2x80x9d which are comprised of individual unit cells of 123 compound which, with reference to each other unit cell of 123 compound in the grain, are in perfect alignmentxe2x80x94i.e., all xe2x80x9cABxe2x80x9d planes of each 123 unit cell are in perfect parallel alignment with each other 123 unit cell comprising that grain. Different xe2x80x9cgrainsxe2x80x9d of 123 may have their xe2x80x9cABxe2x80x9d plane out of perfect parallel alignment therebetween. The degree of such misalignment between the AB planes of adjacent grains of 123 may be very slight or very great. High angle grain boundariesxe2x80x94i.e., those wherein there is a high degree of misalignment of the intergranular xe2x80x9cABxe2x80x9d planes between adjacent grains is a xe2x80x9cweak linkxe2x80x9d (1).
The intervention of a non-superconducting composition between xe2x80x9cABxe2x80x9d planes of one grain of 123 to that of another grain is yet another form of xe2x80x9cweak link.xe2x80x9d Superconduction across the non-superconducting composition between the adjacent 123 grains can still occur, by the xe2x80x9cJosephson (or tunnelling) effect,xe2x80x9d in the absence of an externally applied magnetic field. However, such intervening insulator xe2x80x9cimpurity weak linkxe2x80x9dxe2x80x94depending upon its dimensionsxe2x80x94quickly become electrically resistive in the presence and in proportion to the extent of an externally applied magnetic field.
Such xe2x80x9cinsulatingxe2x80x9d weak links may be comprised of an xe2x80x9cinsulator impurity compositionxe2x80x9d which occupies the physical space between adjacent grains of 123 HTS or may be physical voids between such grains. When an insulator composition occupies such space, the xe2x80x9cweak linkxe2x80x9d is referred to as a xe2x80x9csecondaryxe2x80x9d or xe2x80x9cimpurityxe2x80x9d phase (3). When such weak link is a void of any material, it is referred to either as xe2x80x9cporosityxe2x80x9d (4) or as a xe2x80x9cmicrocrackxe2x80x9d (2), depending upon the nature of the processing circumstances by which such void in a body of the 123 HTS compound came into being.
Another form of xe2x80x9cweak linkxe2x80x9d may occur intragranularly wherein, although the AB planes of the 123 compound comprising such grain are perfectly parallel aligned, some portions of such AB planes are imperfectly oxygenated. That is, within a parallel alignment series of AB planes of individual unit cells comprising a 123 grain, some or a series of such aligned unit cells are comprised of a cell formula wherein the oxygen content is less than about 6.7 and hence such unit cells are of a nonsuperconducting tetragonal crystalline symmetry. Such xe2x80x9cweak linksxe2x80x9d are referred to as of the xe2x80x9clocal or global deficiency of oxygenxe2x80x9d kind (5).
The last form of weak link is associated with the different degrees of capability of a 123 grain to carry current in differt directions, e.g., easier along the AB-plane than perpendicular to the AB-plane. The misalignment of the AB-plane will force current to flow in part, no matter how small, within the 123 HTS body along the C-axis which will act as a weak-link to limit Jc. Each of the above weak link factors reduces the amount of current (I) which is able to flow from one end of a 123 HTS body to another end of the 123 HTS body before the body begins to exhibit an electrical resistance. One of the most significant weak link sources is high angle misaligned grain boundaries.
Weak links in the form of lack of densityxe2x80x94i.e., existence of porosity and/or the existence of micro-cracksxe2x80x94, impurities, or high angle misaligned grain boundaries in a body composed of 123 HTS compound, and the presence of oxygen deficiencies in the grain boundaries, each detract from the amount of current which that body of a 123 HTS compound is capable of carrying before it will exhibit electrical resistance to flow of that xe2x80x9ccriticalxe2x80x9d amount of currentxe2x80x94i.e., the critical current density, (Jc), of that body. A 123 HTS compound, whether produced by solid state reaction, coprecipitation or by a sol-gel technique has, as an intrinsic property, a Tc of xe2x89xa777K. However, the Jc of a body of a 123 HTS compound is highly dependent on the methodology used to produce the 123 HTS compound body. Accordingly, the process by which a body article of 123 HTS is formedxe2x80x94whether that body is in wire, ribbon, film, rod or plate formxe2x80x94dictates many of the practical uses to which that 123 HTS body form may be put, dependent upon the Jc of the body article.
Before the discovery of the 123 HTS compounds allowed use of liquid N2 as a more economical coolant, many possibilities of practical application of superconductivity were unattainable because of the high cost associated with; the use of liquid helium as the coolant required for superconducting materials previously known. Even though HTS compounds have overcome the refrigerant cost barrier which existed against various practical applications of superconductivity, various other practical applications to which a body of HTS compound may be put are, at this time, still limited by the difficulties associated with production of a HTS compound in a body form having a Jc or an ability to carry a large total amount of current (I) required for the practical practice for that particular application. For many applications, the body article must carry a large amount of total current (I). Total current carrying capacity of a body is governed by its cross-sectional dimension and the Jc of the HTS of which that body is composedxe2x80x94i.e., I=Jc(A/cm2)xc2x7cross-sectional dimensions (cm2).
A most serious obstacle to the use of high temperature superconductors (HTS""s) for large current applications is the limited total current carrying capacity of these HTS""s in bulk body forms. The limited Jc of the HTS compounds of which a body is composed seems to be consistent with the small pinning potential associated with the small coherent length of HTS materials. See, Y. Yeshurun et al., Phys. Rev. Lett. 60 2202-2205 (1988). However, experiments have shown that a Jc of up to 5xc3x97106 at A/cm2 exists in 123 HTS films at O T and 77K and 5xc3x97105 A/cm2 in intragrains at 0.9 T and 77 K when 123 HTS compounds are properly processed. See R. K. Singh, et al., Appl. Phys. Lett., 54 2271-2273 (1989) and R. B. van Dover, et al., Nature, 342 55-57 (1989).
To overcome some of the weak link problems in order to enhance Jc, one approach has been to use a melt-texturing technique to form bodies of a 123 HTS compound. See T. Aselage, et al., J. Mat. Res., Vol. 3, pp. 279-291 (1988) and, M. Murakami, et al., Jap. J. Appl. Phys., Vol. 28, pp. L399-L401 (1989). It is known that Y1Ba2Cu3O6+xcex4 undergoes the transition of (Y2O3+liquid)xe2x86x92(Y2BaCuO5+liquid)xe2x86x92(Y1Ba2Cu3O6+xcex4+liquid of BaCuO2 and CuO) as it cools from a single-phase liquid region to xcx9c900xc2x0 C. Near xcx9c1050xc2x0 C., Y1Ba2Cu3O6+xcex4 forms through a peritectic solidification according to Y2BaCuO5+liquid (3BaCuO2+2CuO)xe2x86x922Y1Ba2Cu3O6.5. The peritectic temperature for a L1Ba2Cu3O6+xcex4 compound is that temperature at which its constituents; namely, L2Ba1Cu1O5 (solid phase)+a liquid phase comprising 3BaCuO2 and 2CuO, react to form L1Ba2Cu3O6+xcex4. For a LiBa2CU3O6+xcex4 compound wherein L is Y, the peritectic temperature range has been reported to be about 1020-1050xc2x0 C, with peritectic solidification to produce Y1Ba2Cu3O6+xcex4 beginning at 1020xc2x120xc2x0 C. and continuing as the composition is cooled to about 950xc2x130xc2x0 C. See, for example, Terry Aselage and Keith Keefer J. Mater. Res., 3(16) November/December 1988, pp. 1279-1291.
It is also known that heating above the solidus temperature helps dissolve the impurity phases which are precipitated in the grain boundaries, and that crystal grains of 123 compound grow more easily in a liquid solution. Heretofore, all reported melt-textured methods to generate 123 HTS compound bodies of large Jc consist of rapid heating a body composed of a Y1Ba2Cu3O6+xcex4 compound above the solidus temperature (or even above the melting point) of the compound followed by slow cooling of the body through the xe2x80x9cperitectic temperaturexe2x80x9d of the compound. This has been achieved by a prescribed temperature schedule i.e., certain time (t) rates of temperature (T) variation (dT/dt). The temperature schedule has been shown to be critical to the morphology grains of the 123 compound obtained in the body article. For instance, both needle-like and platelet grain formations have been obtained in bodies composed of Y1Ba2Cu3O6+xcex4. See S. Jin, et al., Phys. Rev. B., 37, 7850-7853 (1988) and P. J. McGinn et al., Physica C, 156, 57-61 (1988).
In a melt-textured growth process for preparing bodies of Y1Ba2Cu3O6+xcex4 all portions of the entire body are simultaneously first elevated in temperature beyond the peritectic temperature of the compound to incongruently melt the 123 compound into Y2BaCuO5 and a liquid phase. Thereafter the body is allowed to cool, at a controlled rate (dT/dt), to a temperature below the solidus temperature wherein the Y2BaCuO5 and liquid phase react to reform the 123 compound which crystallizes. In the melt-textured growth process reported by Jin et al. in Appl. Phys. Lett., Vol. 52, pp. 2074-2076(1988) and Vol. 54, pp. 584-586 (1989) and Murakami et al. in Jpn. J. Appl. Phys., Vol. 28, pp. L1125-1127 (1989) the entire body of 123 compound is first heated above the solidus or peritectic temperature and thereafter the entire body is subjected to a thermal gradient to promote directional solidification of the 123 grains as the body is permitted to cool at a controlled rate to a temperature below the solidus temperature.
By use of a xe2x80x9cmelt textured growthxe2x80x9d process, a number of groups have reported making 123 compound bodies of enhanced Jc To date the highest reported Jc of a 123 body prepared by a melt textured growth technique is 1.7xc3x97104 A/cm2 by Jin et al., Phys. Rev. B. Vol. 37, pp. 7850-7853 (1988) and Appl. Phys. Lett., Vol. 42, pp. 2074-2076 (1988); later Salama et al. employed a liquid-phase processing method to obtain a Jcxcx9c7.5xc3x97104 A/cm2; both in bulk bodies of YBa2Cu3O7xe2x88x92xcex4 (Y123) at 0 T and 77 K after prolonged oxygen annealing. In each method of body treatment the enhanced Jc has been ascribed to the drastically improved grain alignment, phase purity, and densification in Y1Ba2Cu3O6+xcex4 compound of which the body is composed.
In a second approach for enhancing the Jc of a HTS compound body fast neutron irradiation of a body composed of sintered polycrystalline HTS and of single crystals of HTS has been examined for its effects upon the superconducting properties of such materials. A. Wisniewski, et al., Solid State Communications, Vol. 65 (1988) 577-580; H. Fxc3xcpfer et al., Z. Phys. B. 69 (1987) 167-171; Cost et al., Phys. Rev. B. 37 (1988) 1563-1568, report that fast neutron irridation of sintered polycrystalline bodies of Y1Ba2cu3O7 decreases the onset transition temperature of such bodies while increasing the Jc.
Fast neutron irradiation of single crystals of Y1Ba2Cu3O7 has been reported to increase Jc without significant adverse affect on Tc. See Umeqawa et al., Phys. Rev. B. 36 (1987) 7151-7154. Although it has been reported that nominally identical fast neutron irradiations have resulted in effects on critical current ranging from moderate enhancement to degradation, the suggestion has been made that fast neutron irradiation may enhance the Jc at 77K of a body article formed by the melt-textured growth of a HTS such as Y1Ba2Cu3O7. See Van Dover et al., Nature, Vol. 342 (1989) 55-57.
Other forms of neutron radiation of HTS bodies have also been studied. Hence, Hastings et al., J. Am. Ceram. Soc. 71 (1988) C505-506, reports that thermal neutron radiation of a sintered polycrystalline body of Y1Ba2Cu3O7 produced a loss of its 90K superconductive properties, although such 90K super-conducting properties could later be recovered by a post irradiation anneal of such body in flowing O2 at 760K. Fleicher et al., Phys. Rev. B, 40 (1989) 2163-2169, reports that thermal-neutron irradiation of a sintered polycrystalline body of Y1Ba2Cu3O7 doped with uranium enhanced the Jc of the body.
Further efforts to enhance the Jc of a HTS body have been reported by irradiation of such body with high energy protons, Van Dover et al., Information from High Tc Update. Vol. 4, No. 4, p.2, Feb. 15, 1990. But such technique is applicable only to thin films due to the small penetration depth of protons.
In many respects, the enhancement of the Jc of a body comprised of a HTS composition by irradiation with fast neutrons is undesirable since the source for fast neutrons is expensive and can impart to the body a higher than desirable degree of radioactivity. Likewise the use of uranium doped bodies of HTS composition which are irradiated with thermal neutrons to enhance Jc is undesirable because of the long-life radioactivity which would be imparted to such body by such technique.
It is desirable to develop a method by which the Jc of a bulk body composed of a HTS composition, particularly a body produced by melt texturizing, may be enhanced by creating a homogeneous distribution of strong flux pinning centers within such body without imparting thereto an undesirable level of radioactivity.
Enhancement of the Jc of bulk superconductors is a crucial and critical problem for high Tc superconductor applications. This invention comprises a new process to improve the Jc of bulk body articles composed of HTS compositions, particularly bodies composed of the L1M2Cu3O7 HTS compositions and the bismuth and thallium HTS compositions, by thermal neutron induced nuclear reaction in the superconductors doped by 6Li or 10B. The advantages of the process of this invention are simplicity, uniformity for bulk material processing, no long life-time radioactivity to be imparted to the superconductor body, and economy of the process.
A thermal neutron flux is a simple and relatively inexpensive particle source. Thermal neutron irradiation is a very uniform process in the bulk volume of a body composed of a HTS composition due to the very long penetration depth of thermal neutrons. The uniform irradiation of bodies composed of 6Li or 10B doped HTS compositions is an effective technique for modification of body articles thereof because of the large thermal neutron cross-section for 6Li (945 barns) and 10B (4010 barns). The thermal neutron induced reactions which the 6Li and 10B dopants within the body undergo are:                                                        6                    ⁢          Li                +                  n          ⁢                      xe2x80x83                    ⟶                                                                                       3                        ⁢            H                    ⁢                      xe2x80x83                    ⁢                      (                          2.73              ⁢              MeV                        )                          +                                                           4                        ⁢            He                    ⁢                      xe2x80x83                    ⁢                      (                          2.05              ⁢              MeV                        )                                                                                     10                    ⁢          B                +                  n          ⁢                      xe2x80x83                    ⟶                                                                                       7                        ⁢            Li                    *                      (                          0.84              ⁢              MeV                        )                          +                                                           4                        ⁢            He                    ⁢                      xe2x80x83                    ⁢                      (                          1.47              ⁢              MeV                        )                                                  ⟶                                                                               7                        ⁢            Li                    ⁡                      (                          1.01              ⁢              MeV                        )                          +                                                           4                        ⁢            He                    ⁢                      xe2x80x83                    ⁢                      (                          1.78              ⁢              MeV                        )                              
As a result of the long penetration depth of thermal neutrons into body articles composited of such superconductor compositions and the uniform doping of 6Li and 10B into these materials the distribution of the nuclear reaction and the flux pinning defects induced by the reaction products are very evenly distributed in the bulk materials of the HTS composition which comprise the body articles which are so treated.
The body article of HTS composition to which this process applies is one which contains 6Li or 10B in intimate and homogeneous admixture with the HTS composition. The dopant D, which is Li or B, may be incorporated into the HTS composition in either of two ways, as a unit cell external or as a unit cell internal dopant. In the former case the precursor composition comprises an intimate mixture of the L, M and Cu reagents in proportions which provide a ratio of L:N:Cu of 1:2:3 and this mixture further comprises a quantity of the Li or B reagent such that upon sintering an HTS composition of a L1M2Cu3O7 unit cell formula results. In this case, the Li or B content of the composition is dispersed at positions external to the L1M2Cu3O7 unit cells, such as at the grain boundaries between different grains of the 123 HTS composition which comprises the body article. In the unit cell internal method for incorporation of the dopant D, which is either Li or B, the L, M, Cu and D reagents are intimately mixed in proportions which provide for a ratio of metal atoms of the formula
(L+M)3-zDzCu3
wherein z is greater than zero and equal to or less than 0.3 and the atomic: ratio of L:M is from about 0.35 to about 0.6. When the precursor mixture is sintered or melt texturized to convert it into a HTS composition the product composition which results is of the formula
(L+M)3-zDzCu3O6+d
where d is 0.1 to 1.0, and the dopant atom D is incorporated into the 123 unit cell structure wherein it occupies either a L or a M atomic site. In the case of a Bi or Tl HTS composition, i.e., those of the formula T2Mxe2x80x22CanCun+1O6+2n, the dopant D may also be externally or internally incorporated. When internally incorporated the precursor reagents are mixed in proportions to provide a product composition, after sintering or melt texturizing, of the formula
T2Mxe2x80x22Can(Cu1-zxe2x80x2Dzxe2x80x2)n+1O6+2n
wherein zxe2x80x2 is greater than zero and equal to or less than 0.5.
The method of internal incorporation of the dopant D is preferred since this method is more adapted to the preparation of high quality HTS body articles by melt texturizing, i.e. greater grain alignment with less intergranular impurity weak link content. A dopant most preferred is Li, particularly preferred is a lithium which is enriched in 6Li. The 6Li or 10B containing HTS 123 composition may be prepared in a body article form by sintering or melt texturizing of preshaped bodies of intimately mixed L2O3, MCO3 and CuO powders to which has been added and mixed Li2Co3 or B2O3 to the level of 6Li or 10B desired. As a preferred alternative to simple sintering, the body article is treated by a melt texturizing process to produce a high degree of grain alignment in the body. Bismuth and thallium HTS bodies are prepared in like manner.
The irradiation of such 6Li or 10B doped bodies of HTS composition by thermal neutrons causes the 6Li or 10B content thereof to undergo an induced nuclear reaction. The energetic light particles from the nuclear reaction produce radiation damage and defects with the HTS composition of the body which uniformly introduces strong flux pinning centers within the bulk volume of the body, thereby substantially enhancing the Jc of the body compared to its unirradiated state. For highest attainable Jc it is preferred to practice this invention with 6Li or 10B internally doped articles that are prepared by a melt texturizing technique.
The invention comprises 6Li or 10B doped HTS compound, preferably a 123 HTS compound, having a predetermined body shape which, after irradiation with thermal neutrons, has a current density, Jc, of from about 103 to about 105 amps/cm2 or greater at zero magnetic field and a temperature of at least 77K.
The preferred 123 HTS composition to which the process of this invention is applicable is internally doped with Li to prepare preshaped 123 HTS compound body articles is (L+Ba)3-zLizCu3O7, most preferably wherein L is yttrium. The preferred T2Mxe2x80x22CanCun+1O6+2n HTS composition is one wherein T is bismuth and Mxe2x80x2 is strontium and the HTS body is internally doped with Li to provide a formula of Bi2Sr2Can(Cu1-zxe2x80x2Lizxe2x80x2)n+1O6+2n. The preferred dopant is one of lithium, most preferred is a lithium which is enriched in the 6Li isotope.